Software select and test

ABSTRACT

A software-controlled select and test system for use with a spacecraft. The system reduces the assembly, integration, test time and improves performance of the payload and spacecraft by optimizing the performance using standardized algorithms. The system also reduces the DC power required by the spacecraft, and the thermal dissipation, battery size, solar array size and mass of the spacecraft. This is achieved by optimizing traveling wave tube amplifiers for each channel instead of meeting minimum power requirement over the entire range, wherein most channels have excess power. A serial interface adapter performs the optimization functions via software.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior application Ser.No. 08/891,491, filed on Jul. 11, 1997 now U.S. Pat. No. 5,781,456.

BACKGROUND

The present invention relates generally to select and test systems, andmore particularly, to a software-based select and test system for usewith spacecraft.

Heretofore a large portion of the time allocated to payload andspacecraft integration involves manual adjustments and hand selection ofalternative components that optimize the performance of analog and RFcircuits. This operation is referred to as a select and test operation.Conventional select and test operations use factory selected componentsinstalled during assembly, integration and test or use a limited numberof switched components. The select and test operation requires speciallyskilled personnel and expensive test equipment. An error orout-of-specification response encountered during an acceptance testoperation requires replacement of the out-of-specification circuits andretesting of the payload to high reliability standards due to the factthat the electrical path was broken by making a hardware change.

It would therefore be an advantage to have a software-based select andtest system for use with a spacecraft that eliminates the need toreplace or manually solder components and attenuators to implementselect and test operations, and has the added advantage of reducing thepower and thermal dissipation requirements of the spacecraft.

SUMMARY OF THE INVENTION

To meet the above and other objectives, the present invention providesfor a software-based or software-controlled select and test system thatprovides two major improvements when it is employed with a spacecraft.The software-based select and test system reduces the assembly,integration, test time and improves performance of payload andspacecraft. The time savings is achieved by eliminating the need toreplace coaxial attenuators and manually solder components, by usingcomputer-controlled optimized performance established by standardizedalgorithms.

The software-based select and test system also provides for reducedthermal dissipation, battery size, solar array size and mass resultingfrom the five percent reduction in required payload power. This savingsis achieved by optimizing traveling wave tube amplifiers for eachspecific channel instead of meeting minimum power requirement over theentire range, wherein most of the channels have excess power. Priortraveling wave tube amplifiers did not have this flexibility for aprecise change in RF power achieved using anode control as is providedby the present invention.

A serial interface adapter has been developed by the assignee of thepresent invention provides the capability to economically perform theoptimization functions using software. The present inventiondramatically increases the number of spacecraft that can be inventoriedand by reducing the payload power by approximately five percent byimproving the payload performance on a detail level that is not possiblewith manual methods.

Thus, the present select and test system replaces fixed select and testresistors with computer controlled operational amplifiers that providebreakpoints, variable gain and temperature compensation functions. Powerand frequency optimization is provided by the present invention.Significant improvements in payload performance are achieved byoptimizing for a specific channel over temperature instead of optimizingpayload performance over the entire bandwidth prior to spacecraftintegration, which involves many compromises.

On a spacecraft that is restricted to a limited number of channels basedon placement of traveling wave tubes, improved performance is achieveddue to the reduced range for that location and is then reset foralternative channels. A key element of this is the power from thetraveling wave tube amplifier which may be adjusted by changing itsanode voltage. Limited frequency changes may also be implemented bychanging the cathode voltage. The traveling wave tube amplifier istypically set for the worst case to meet specification which is notrequired for the majority of the channels, There is typically a 0.4 dBvariation between the center of channel and the end. Reducing the powerof the traveling wave tube amplifier for channels in the center reducesthe payload power by approximately five percent (0.21 dB).

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 illustrates a conventional hardware select and test circuit; and

FIG. 2 illustrates a software-based select and test circuit inaccordance with the principles of the present invention;

FIG. 3 illustrates a conventional RF select and test circuit;

FIG. 4 illustrates an RF select and test circuit in accordance with theprinciples of the present invention;

FIG. 5 is a graph illustrating the RF response versus frequency of atraveling wave tube amplifier,

FIG. 6 illustrates a conventional traveling wave tube amplifier anodecontrol circuit, and

FIG. 7 illustrates a traveling wave tube amplifier anode control circuitin accordance with the principles of the present invention.

DETAILED DESCRIPTION

Referring to the drawing figures, FIG. 1 illustrates a conventionalhardware select and test circuit 10. The hardware select and testcircuit 10 includes a resistor 11 and Zener diode 12 serially coupledbetween a voltage source and ground. Two serially coupled variableresistors 13a, 13b are coupled from a point between the resistor 11 andZener diode 12 and ground. An operational amplifier 14 is coupled to apoint between the variable resistors 13a, 13b. The output of theoperational amplifier 14 provides an adjusted operating point resultingfrom manually adjusting the variable resistors 13a, 13b.

Electronic units disposed on a spacecraft (generally designated asspacecraft 16) require select and test alignment to establish theappropriate gain, temperature compensation and bias that provides foroptimum performance of the electronic units. The electronic unitsinclude but are not limited to receivers, upconverters, downconverters,low noise amplifiers, linearizers, solid state power amplifiers,traveling wave tube (TWT) amplifiers (TWTA) in the payload and theamplifiers, comparitors, motor drivers, heater drivers, etc. in the busequipment. Conventionally, such alignment is achieved by manuallyadjusting the variable resistors 13a, 13b. A typical select and testoperation involves at least one and generally two select and testdevices (the variable resistors 13a, 13b). Analog circuits are generallymodified for operation using an adjustable voltage.

Referring now to FIG. 2, it illustrates a software-based select and testcircuit 20 in accordance with the principles of the present invention. Aserial interface adapter (SIA) 21 developed by the assignee of thepresent invention provides the capability to economically perform selectand test functions in software. The serial interface adapter 21 has tenprecision-controlled pulse width modulators (PWM) 22 that can vary theiroutput in 1/1024 increments. The serial interface adapter 21 alsoincludes a precision voltage reference 23 (Zener diode). The output ofthe serial interface adapter 21 is provided by a digital to analogconverter (D/A) 28, when filtered using filter 24, such as is providedby a resistor 25 and capacitor 26, for example, provides for a precisionsoftware-controlled output voltage which is applied to an operationalamplifier 27. The output of the operational amplifier 27 is an adjustedoperating point for a selected electronic unit.

The select and test values for the electronic unit are automaticallyestablished by automatic test equipment coupled to the serial interfaceadapter 21 that varies the values for optimum performance. This totallyeliminates the need for manual soldering or manual adjusting ofcomponents in the electronic unit to provide for select and testfunctions.

Referring now to FIG. 3, it illustrates a conventional RF select andtest circuit 30. At the spacecraft and panel, the conventional RF selectand test operation includes making RF measurements and changing manuallyadjusted attenuators 32 to obtain the optimum performance and to preventexcessive power from destroying an electronic unit. A basicimplementation is shown in FIG. 3, where the outputs of two receivers 31are balanced using RF attenuators 32 coupled to outputs of therespective receivers 31. The receivers 31 are adjusted to have the samegain so that the electronic unit operates the same when either one ofthe receivers 31 is used.

FIG. 4 illustrates an RF select and test circuit 40 in accordance withthe present invention. The RF select and test is performed by softwarewhen an attenuator 32a is incorporated n the RF unit. An RFcomputer-controlled attenuator 32a is employed in channel amplifiersused in the present invention. The serial interface adapter 21 performsthe gain adjustment function by way of a serial bus that was previouslyperformed by manually replacing alternative coaxial attenuators. Theaddition of the computer-controllable attenuators 32a in the receiver31a allows the entire system to be optimized, thus improving performanceover the previous operation. It is also extremely fast and does notrequire the RF path to be broken in order to establish to optimumperformance during assembly integration and test alignment.

Almost all of the payload power for a typical communication satellite orspacecraft 16 is established by the DC power supplied to traveling wavetube amplifiers used in the payload. The traveling wave tube amplifiersare designed to provide a minimum value of output power over apredetermined frequency range. Electronic units also have lowerinsertion loss than the specified value so that the satellite orspacecraft 16 typically has a higher power output than is required, andwhich is on the order of at least 0.1 dB total.

FIG. 5 shows a typical graph of output power versus frequency for atraveling wave tube amplifier employed in a communications link. Theoutput power is typically 0.4 dB higher than is required in the centerof the band. If it is assumed that the average is 0.2 dB higher over theband, 4.5 percent extra power may be gained by lowering the power outputof the traveling wave tube amplifier to the specification level at thefrequency of operation. This value increases to 5 percent if theelectronic units are on average better than specification. The savingsderived from this change is about 3 million dollars for a typically highpower communications satellite.

The power can be lowered if the requirement is to meet the specificationusing the redundant electronic unit. This typically requires a higherpower in the primary path to assure adequate power in the redundant pathwhich has higher losses due to extra switches and waveguide or coaxialfeeds, for example. On the other hand, there are times when the lossesare greater than anticipated and the spacecraft 16 does not meet thespecification by a small amount. This requires changing the electronicunit or generating paperwork to justify the delivery of an electronicunit with less than specified performance. This problem may be resolvedby this computer controlled anode control.

The traveling wave tube amplifier incorporates an anode regulator. TheRF power output of the traveling wave tube amplifier is mathematicallyproportional to beam current. The anode regulator senses the beamcurrent to maintain a constant power output. The output power thereformay be adjusted by changing the beam current. FIG. 6 shows a blockdiagram of a conventional anode control circuit 60 of a traveling wavetube amplifier, which is similar to the conventional select and testcircuit 10 used to adjust the reference voltage. Conventional travelingwave tube amplifier adjustments have been primitive, typically allowingonly a one level change by incorporating additional transformers. Thisarrangement produces a large change (typically 3 dB) in output.

FIG. 7 illustrates a software-based traveling wave tube amplifier anodecontrol circuit 70 in accordance with the principles of the presentinvention. The anode control circuit 70 comprises a serial interfaceadapter (SIA) 21 described above with reference to FIG. 2. The output ofthe serial interface adapter 21 is filtered using a filter 24, providedby the resistor 25 and capacitor 26, for example, produces a precisionsoftware-controlled output voltage that is applied to the operationalamplifier 27. The output of the operational amplifier 27 is an adjustedoperating point for a selected electronic unit. The output of the filtercircuit 24 provides a bias voltage (V_(ref) bias) that is also employedin the anode control circuit 70 that augments the reference voltage(V_(ref)), thus allowing the RF power output to be changed. Selectionand control of the operating point (i.e., the RF power output) isperformed at a ground station by way of the serial interface adapter 21.

Thus, a software-based or software-controlled select and test system foruse with spacecraft has been disclosed. It is to be understood that thedescribed embodiment is merely illustrative of some of the many specificembodiments which represent applications of the principles of thepresent invention. Clearly, numerous and other arrangements can bereadily devised by those skilled in the art without departing from thescope of the invention.

What is claimed is:
 1. A software-controlled select and test system foruse with a spacecraft, characterized by:said spacecraft comprising aplurality of controllable units; a plurality of interface adapters eachcomprising circuitry that varies its output in a controlled manner inresponse software-controlled inputs, and wherein control of thecircuitry provides an output that is used to adjust the controllableunits to optimize the performance thereof.
 2. The select and test systemas recited in claim 1, wherein the plurality of serial interfaceadapters each comprise a plurality of controllable pulse widthmodulators that vary their output in a controlled manner in responsesoftware-controlled inputs, and wherein control of the pulse widthmodulators provides for an output voltage that is used to adjust arespective electronic unit to optimize the performance thereof.
 3. Theselect and test system as recited in claim 1, comprisinga plurality ofamplifiers as electronic units; a plurality of control circuits thateach comprise a control voltage input for receiving a control voltagefrom said pulse width modulator of the serial interface adapter and anoutput that adjusts an operating point of the respective amplifier. 4.The select and test system as recited in claim 1, comprisinga pluralityof receivers that are individually coupled to a respective plurality ofcomputer-controllable attenuators; and wherein control of the pulsewidth modulators of each serial interface adapter provides for an outputvoltage that adjusts an operating point of the computer-controllableattenuator that in turn adjusts the gain of the respective receiver. 5.The select and test system as recited in claim 1, comprisinga pluralityof traveling wave tube amplifiers as electronic units; a plurality ofanode control circuits that each comprise a reference voltage input forreceiving a reference voltage, a cathode current sense input for sensingthe current supplied to the cathode of the amplifier, and a controlvoltage input for receiving a control voltage; said plurality of serialinterface adapters being individually coupled to the respective anodecontrol circuits, and wherein each serial interface adapter outputs acontrol voltage that is coupled to the respective control voltage inputof the anode control circuit that adjusts an operating point of theanode the respective traveling wave tube amplifier; and a bias voltagerespectively coupled to the control voltage input of the respectiveanode control circuit that augments the reference voltage to vary the RFpower output of the respective traveling wave tube amplifier; andwherein RF power output is controlled by way of the serial interfaceadapter.
 6. The select and test system as recited in claim 1, whereinthe controllable electronic units are controlled from a ground stationby way of the serial interface adapter.
 7. The select and test system asrecited in claim 3, wherein the operating point of each respectiveamplifier is adjusted using select and test values transmitted from aground station by way of the serial interface adapter.
 8. The select andtest system as recited in claim 5, wherein the gain of each respectiveamplifier is adjusted using select and test values transmitted from aground station by way of the serial interface adapter.
 9. The select andtest system as recited in claim 4, wherein the RF power output of eachrespective amplifiers is adjusted using select and test valuestransmitted from a ground station by way of the serial interfaceadapter.